Contact angle measurements are commonly used to characterize the surface properties of solid or liquid materials. The value of the contact angle between the drop of liquid and the solid or liquid substrate on which the drop rests depends on the forces occurring at the liquid-solid or liquid-liquid interface.
These measurements are applicable not only to liquid substrates for measuring the interface between two liquids, but also with solid substrates for measuring their interface properties, and consequently for measuring their wetting properties.
If perfect and complete wetting takes place, the drop of liquid spreads out over the substrate and the contact angle is zero, whereas if wetting is only partial, the resulting contact angle lies in the range 0.degree. to 180.degree..
A drop of liquid water surrounded by its vapor may be taken as an example. The drop is placed on a solid substrate and the contact angle .theta. on the boundary line between the three phases, i.e. on the triple line, has a single value for smooth homogeneous and isotropic substrate surfaces. This value is related by Young's equation: EQU .gamma..sub.SV -.gamma..sub.SL =.gamma..sub.LV cos .theta.
to the free energy .gamma..sub.SV at the solid-vapor interface, to the free energy .gamma..sub.SL at the solid-liquid interface, and to the tension .gamma..sub.LV at the liquid-vapor interface. Zisman emphasized the concept of a critical surface tension for wetting, .gamma..sub.C, which is a characteristic of each solid surface. Thus, glass and metals are examples of high energy surfaces over which most liquids spread spontaneously, with the angle .theta. tending to zero. In contrast, plastic materials such as tetrafluoroethylene are typical low energy surfaces such that liquids placed on these surfaces remain in the form of drops having finite contact angles so long as .gamma..sub.C is less than .gamma..sub.LV.
Measurements of these contact angles are used not only for fundamental or applied research, but also in industry for performing routine tests on the surface states of certain materials.
Such measurements are of interest to very many enterprises engaged in a wide range of activities.
For example, enterprises which perform surface state tests on different kinds of material are concerned, in particular the surface states of the following materials are commonly tested: plastic and polymer films; wetting agents and detergents; textiles; inks; glues; composite materials; and special materials such as those used in the biomedical field for contact lenses, dentistry, etc. Another activity which is closely concerned with the surface state of material is depositing thin layers on a substrate for modifying its surface properties, for example depositing a thin layer of tetrafluoroethylene on metal drums for use in offset printing. Other industries are also concerned, for example the oil industry.
At present there are several experimental procedures for measuring contact angles.
Most of these procedures are based on observing a drop placed on a substrate, thus having the advantage of requiring very small quantities of liquid and very small areas of substrate surface, e.g. only a few square millimeters.
In the most widely used procedures, a "silhouette" image of the drop is projected and the drop angle is measured point by point using a telescope and a protractor, which requires a tangent to be estimated by eye. Such procedures are not very reproducible, and a reproducibility of less than .+-.2.degree. is commonly observed when comparing readings obtained on successive drops or on several different projection views of the same drop.
This lack of reproducibility is made worse by local roughness or lack of uniformity on the solid substrate being used. Given that this problem is inherent to the substrate itself, it will always be present however much care is taken by the experimenter. The only way of reducing dispersion in the results is thus to take the average of a large number of individual measurements, which naturally constitutes a particularly boring and difficult task.
In another known method, interference fringes are created in a liquid wedge formed at the edge of the drop and the contact angle is deduced from the fringe spacing. However, this method is applicable only to determining very small contact angles.
Another measuring procedure consists in autocollimating light very locally on the edge of the drop by means of a beam of light whose angle of incidence relative to the substrate is varied. The intensity of the beam reflected by the drop is observed in order to detect the moment when the beam is extinguished. The position of the incident beam angle can then be used to immediately deduce the contact angle between the drop and the substrate. The drawback with this method, as with the preceding methods is that it requires the measurement to be performed on a particular portion of the drop, thereby leading to poor measurement reproducibility.
Preferred implementations of the present invention avoid the above-mentioned drawbacks since the contact angle between a drop which is placed or "sessile" on the substrate is determined simultaneously over the entire periphery of the drop.
Contact angles can then be determined quickly and accurately, with a typical accuracy of 0.1.degree. rather than 1.degree. to 2.degree. as applicable to the prior arts methods.
The method also provides objective measurement in that it does not require a tangent to the drop to be estimated by eye at the point of contact, thereby avoiding possible variations from one observer to another.
The method in accordance with the invention does not require an optical microscope to be used as is generally the case with prior art methods, it enables badly formed drops to be visually detected immediately, which is generally impossible in prior art methods and constitutes yet another source of erroneous measurements.
The method in accordance with the invention also constitutes an immediate test for substrate uniformity on the basis of the shape of a drop (drop asymmetry, or irregular contour).
Further, the method in accordance with the invention is capable of being automated so that measurements are performed continuously, given that the measurement is particularly simple to implement, as will be seen from the description below.